This invention pertains to isotropic, negative thermal expansion ceramics, and to a process for preparing isotropic, negative thermal expansion ceramics which may, for instance, be used in temperature compensating grating packages.
As described in U.S. Pat. No. 5,694,503, incorporated herein by reference, it is possible to package a fiber grating on or in a negative expansion material so that the package and grating dimensions decrease with an increase in temperature, resulting in minimal variation of the reflection wavelength with temperature.
In order for this approach to be practical the value for the thermal expansion of the package/substrate material must lie within an acceptable range. The ideal expansion coefficient is given by the expression       α    ideal    =            α      fiber        +                  -        A                              (                      1            -                          P              e                                )                ⁢                  xe2x80x83                ⁢                  λ          nom                    
where xcex1fiber is the thermal expansion of the fiber grating (for instance, 0.55xc3x9710xe2x88x926xc2x0 C.xe2x88x921), A is the temperature sensitivity of the unpackaged grating (e.g., 0.0115 nm xc2x0C.xe2x88x921 for a particular 1550 nm grating), Pe is the photoelastic constant (typically 0.22) and xcexnom is the nominal grating wavelength (xcx9c1550 nm in many cases). For a particular 1550 nm grating an xe2x80x9cidealxe2x80x9d package material would have a thermal expansion coefficient (CTE) of xe2x88x928.96xc3x9710xe2x88x926xc2x0 C.xe2x88x921. A package material will still be beneficial even if its thermal expansion is not exactly equal to the ideal value. For the above assumptions a factor of about 20 improvement in temperature sensitivity would be achieved if the package material""s thermal expansion coefficient were within 0.47xc3x9710xe2x88x926xc2x0 C.xe2x88x921 of the ideal value. Such improvement in thermal stability of a fiber grating would be of commercial importance.
It is known that ZrW2O8 is metastable at room temperature, with the lower limit of stability being at 1105xc2x13xc2x0 C., below which ZrW2O8 decomposes into ZrO2 and WO3. See, for instance, J. Graham et al., J. American Ceramics Society, Volume 42, page 570 (1959), and L. L. Y. Chang et al., J. American Ceramics Society, Volume 50, page 211 (1967). It is also known that ZrW2O8 has a relatively high and isotropic negative coefficient of thermal expansion (CTE) over an extensive range of temperatures that includes room temperature. See, for instance, C. Martinek et al., J. American Ceramics Society, Volume 51, page 227 (1968) and T. A. Mary et al., Science, Volume 272, page 90 (1996). Specifically, the CTE is substantially constant from near absolute zero temperature to 150xc2x0 C., with a value near xe2x88x9210xc3x9710xe2x88x926xc2x0 C.xe2x88x921. The material exhibits an order-disorder transition at 150xc2x0 C., after which the CTE drops to xe2x88x925xc3x9710xe2x88x926xc2x0 C.xe2x88x921. This value of the CTE is maintained until the decomposition of ZrW2O8 which occurs at a relatively high rate near 800xc2x0 C.
In view of this complex behavior of ZrW2O8, it is not surprising that earlier attempts to produce mechanically strong monolithic bodies of ZrW2O8, that have a predetermined negative CTE, did not yield fully satisfactory results. For instance, C. Verdon et al., Scripta Materialia, Volume 36, page 1075 (1997) report that their attempts to form electrically conducting bodies from ZrW2O8 and Cu with a low CTE resulted in decomposition of the ZrW2O8 and formation of Cu2O along with other compounds. Such decomposition is generally undesirable and hinders the production of suitable bodies.
To the best of our knowledge, prior art efforts to make ZrW2O8 ceramic bodies used the conventional technique of sintering ZrW2O8 powder. Thus produced bodies have densities less than 90% of the theoretical density and exhibit relatively poor mechanical properties, specifically, a low modulus of rupture. Additional prior art describes the preparation of ZrW2O8 powder from oxide precursors to be an incomplete reaction.
In view of the importance, for instance, a reduction of the temperature dependence of the reflection wavelength of optical fiber gratings, it would be highly desirable to be able to reproducibly make mechanically strong ceramic bodies having tunable negative CTE values, the ceramic bodies being useful, for example, for packaging of fiber gratings. This application discloses a method for making such bodies.
We have made the surprising discovery that reactive sintering of appropriate percursor powders (e.g., ZrO2 and WO3) can result in (negative CTE) bodies (e.g., ZrW2O8) with substantially improved properties, as compared to analogous bodies produced by the prior art sintering techniques.
To the best of our knowledge, the prior art does not provide any suggestion that the use of reactive sintering could provide the observed improved results. By xe2x80x9creactive sinteringxe2x80x9d we mean herein compacting the unsintered body of the precursor oxides, rather than the powder of the desired final phase, and then forming the desired phase and densifying the body in a single heat treatment step.
In a broad aspect of the invention is embodied in a method of making negative CTE ceramic bodies, and in bodies produced by the method.
More specifically, the invention is embodied in a method of making a ceramic body having isotropic negative thermal expansion, the body having a major constituent selected from the group consisting of ZrW2O8, HfW2O8, ZrV2O7 and HfV2O7. The method comprises the steps of providing a powder mixture, forming a xe2x80x9cgreenxe2x80x9d body that comprises the powder mixture, and heat treating the green body. The powder mixture comprises a first and a second oxide precursor powder, selected respectively from the group consisting of ZrO2 powder and HfO2 powder, and the group consisting of WO3 powder and V2O5 powder. The heat treatment of the green body includes heating the body to a temperature below the melting temperature of the selected constituent, such that the selected major constituent is formed from the green body by reactive sintering.
By a xe2x80x9cgreenxe2x80x9d body we mean herein the compacted precursor powders as a monolithic body prior to sintering.
In a preferred embodiment the powder mixture has a non-stoichiometric composition (e.g., excess ZrO2), and the heat treatment results in formation of a 2-phase material, e.g., ZrW2O8 majority phase, with ZrO2 inclusions dispersed in the majority phase. This embodiment allows tailoring of the CTE of the body.
In a further preferred embodiment the powder mixture comprises a minor amount of a sintering aid (e.g., Y2O3, Bi2O3, Al2O3, ZnO, TiO2, SnO2), whereby the density of the sintered body is substantially increased.
Important note for processing
Due to the unusually narrow stability region of ZrW2O8, standard techniques for synthesizing and densifying composite ceramics containing ZrW2O8 produce monoliths with inadequate physical properties. High purity ZrW2O8 is thermally stable between 1105 and 1260xc2x0 C. Above 1260xc2x0 C. ZrW2O8 peritectically decomposes into a Liquid phase, and below 1105xc2x0 C. it decomposes into ZrO2 and WO3. Additives significantly alter the stability region of ZrW2O8; this study demonstrates a working temperature range of 1140 to 1180xc2x0 C. for Y2O3 doped ZrW2O8.